An elliptical jacket fiber and a panda fiber, shown in FIGS. 1 and 2 respectively of the accompanying drawings, are known for use as polarizing wave preserving fibers. An ellipical jacket 12 surrounding cladding 11 of the elliptical jacket fiber and stressing members 23 contained within support member 22 and on opposite sides of the cladding 21 of the panda fiber are respectively made from materials which have a high thermal expansion coefficient such as silicon dioxide (SiO.sub.2) glass to which boron (B) or phosphorous (P) are added, consequently aeolotropic stresses bear on cores 13 and 24 possess the property of double refraction, and optical coupling between two perpendicularly-crossed polarized modes is restrained so that only one polarization mode can be transmitted.
In manufacturing the optical fiber, it is generally difficult to avoid fluctuations in the refractive index and the diameter in the longitudinal direction so that optical coupling between two perpendicularlycrossed polarized modes and a high extinction ratio cannot be obtained. Accordingly, the use of such an optical fiber with a sophisticated optical measuring system can result in deterioration of the system.
On the other hand, if the double refractive index of the fiber is raised in order to prevent optical coupling between the perpendicularly-crossed polarized modes from being generated, the dispersions of each polarization are increased, and once the optical coupling occurs, considerable expansion in the pulse width occurs, which is unsuitable for a long-distance communication transmission system.